1. Field of the Invention
The present invention generally relates to optical information reproducing apparatus and more particularly is directed to an optical information reproducing apparatus for reproducing information recorded on a recording surface of an optical disc such as an optical video disc, a so-called compact disc and so on.
2. Description of the Prior Art
Generally, in an optical disc reproducing apparatus, a laser beam is focussed on the recording surface of an optical disc by an objective lens so as to reproduce a signal recorded thereon. In this case, the resolution of the reproducing apparatus is determined by how much the laser beam is converged, that is, by the diameter of the beam spot on the recording surface so that the maximum value of the diameter of beam spot is selected to be less than a certain value. The diameter of the beam spot is determined by the wavelength of the laser light and the ratio between the focal length of an objective lens and its diameter (this ratio is generally presented as an NA (numerical aperture) value) and so on.
By the way, in the art, a helium-neon laser has been used as a light source. This helium-neon laser, however, causes the apparatus to be large in size and expensive. Accordingly, it is a recent trend that a semiconductor laser is used as the light source because it is inexpensive and suitable for making the apparatus compact in size.
However, the semiconductor laser provides light with a wavelength of 780 nm which is longer than the wavelength (623.8 nm) of the helium-neon laser. For this reason, in order to establish a spot diameter of the semiconductor laser which can provide the reproducing apparatus with the same resolution as that using the helium-neon laser as its light source, the NA value of the objective lens must be increased to, for example, about 0.5.
However, if the NA value of the objective lens is increased as described above, when the optical axis of the laser beam is not perpendicular to the recording surface of the disc, crosstalk components from adjacent tracks on the disc become a serious problem.
As shown in FIG. 1A, when an optical axis 2 of a laser beam is perpendicular to the recording surface of a disc 1, a detected output D in its light receiving section or photodetector becomes as shown in FIG. 1A in which crosstalk components from adjacent tracks T.sub.1 and T.sub.2 are small enough relative to the output from a main track T.sub.0. However, as shown in FIG. 1B, when the optical axis 2 of the laser beam is not perpendicular to the recording surface of the disc 1 (this condition will hereinafter be referred to as the skew of the disc 1), the crosstalk components from one of the adjacent tracks T.sub.1 and T.sub.2, in this case, the crosstalk component from the track T.sub.1 in the detected output D, becomes large.
When the NA value becomes large, a crosstalk level Lc can not be neglected as will be clear from the relation expressed as EQU Lc.varies.Wcm.varies.(NA.sup.3 /.lambda.).multidot..theta.
where Wcm is coma aberration, .lambda. is the wavelength of the laser beam and .theta. is a skew angle in the radial direction of the disc. For example, under the conditions that .lambda. is 780 nm, a track pitch is 1.67 .mu.m and NA is 0.5, in order to establish the crosstalk level Lc=-40 dB, the condition of .theta..ltoreq.0.5 has to be satisfied.
The skew of the disc, that is, the lock of perpendicularity of the optical axis of the laser bean relative to the recording surface of the disc, may be caused by the tilting or bending of a spindle shaft, the tilting or bending of a turntable for the disc, the skew or warping of the disc itself and so on. The main cause is the skew of the disc itself, and at present a skew angle .theta. in the radial direction of the disc of 1.degree..ltoreq..theta..ltoreq.2.degree.. For this reason, when the semiconductor laser is used as the light source, it is necessary to detect the overall skew of the disc in the radial direction thereof from all causes (including skews other than the skew of the disc itself) and to cope with the resulting increase of crosstalk components. The term "skew" as used hereinafter will mean skew from all causes.
In order to cope with the increase of crosstalk components, there have been previously proposed the following methods.
In one of such prior art methods, as shown in FIG. 2, in addition to a laser light source included in an optical pick up device 3, a laser 4 is provided as a light source for a skew detector. A bundle of light from this laser 4 is made incident on a disc 1 as a beam of parallel light through a collimator lens 5, a polarizing beam splitter (PBS) 6 and a 1/4 wavelength plate 7 and then reflected from disc 1. The reflected light is returned through the 1/4 wavelength plate 7 and the PBS 6 to a photo detector 8 which is divided into two areas 8A and 8B. In this case, the skew of the disc 1 is detected as the difference between the detected outputs from divided areas 8A and 8B of the divided photo detector 8.
However, this method requires not only the laser 4 as its light source but also the collimator lens 5, the PBS 6 and the 1/4 wavelength plate 7 so that this method is inherently costly. Moreover, since this method uses a beam of parallel light, at the position where the content of the recorded information on the disc 1 is changed or at the position where, in case of the recorded information being, for example, a video signal, the content of the picture is changed, the reflectivity of the disc is changed and, as a result, skew of the disc 1 is erroneously detected.